The invention relates to an electric discharge lamp having
an outer lamp envelope, PA1 a discharge vessel arranged within the outer envelope, the discharge vessel including a pair of discharge electrodes between which a discharge is maintained during lamp operation, opposing seals sealing the discharge vessel in a gas-tight manner, and a conductive feed-through extending from each discharge electrode through a respective seal to the exterior; PA1 frame means for supporting the discharge vessel within the outer envelope and for electrically connecting the discharge vessel to a source of electric potential outside of the outer envelope; and PA1 a containment shield comprised of a light-transmissive sleeve disposed about the discharge vessel and having opposing ends each adjacent a respective seal of the discharge vessel.
Such a lamp is known from U.S. Pat. No. 5,136,204. The purpose of the containment sleeve is to keep the outer bulb intact if the discharge vessel should explode, which may occur when the lamp reaches the end of its life. The presence of the sleeve, however, complicates lamp construction because it must be supported about the discharge vessel. In the commercially available lamp according to this patent, the sleeve is quartz glass and has a wall thickness of 2 mm. Metal clips are secured on the press seals and include portions which hold the ends of the sleeve. The sleeve and discharge vessel are supported by welding the clips to an elongate metal support rod which is fixed around the lamp stem by a metal strap. The support rod, and consequently the metal clips and the sleeve, are electrically isolated which prevents accelerated sodium depletion from the discharge vessel. (For a detailed description of this sodium loss process, reference may be made to the textbook Electric Discharge Lamps by Dr. John Waymouth, M.I.T. Press 1971 (Chapter 10)). As compared to a non-shielded lamp in which the elongate support rod typically extends from the lamp stem or is welded to a stem conductor to carry current to the discharge vessel, the fixing of the support rod to the stem with a metal strap is more expensive and intricate. The clips further add to the number of lamp parts and increase lamp cost.
It is also known from U.S. Pat. No. 4,721,876 to surround the glass sleeve by a meshwork of metal wire which is fixed around the sleeve with metal clamping strips. The provision of the meshwork enables a sleeve of smaller wall thickness to be used while still maintaining adequate containment. The clamping strips are also electrically conducting and connected to a current-carrying conductive support rod of the lamp frame which supports the discharge vessel and the glass sleeve. The meshwork as a result is under electrical tension, which causes accelerated sodium depletion from the discharge vessel as discussed above.
The construction of the '876 lamp is complicated. The manufacture of the meshwork, or of a braided assembly, and its manipulation are difficult. With the clamping strips, the sleeve tends to shift when the lamp is jarred during shipping or handling. Furthermore, the securing of the clamping strips about the meshwork and sleeve requires expensive banding equipment and/or hand welding to the lamp frame which increases lamp cost.
It is also known to use outer lamp bulbs which have a larger wall thickness than standard bulbs to protect against failure in the event of a discharge vessel explosion. However, using such non-standard bulbs greatly increases lamp cost.